The invention relates to a method of checking a capacitive actuator, particularly an actuator for a fuel-injection valve in an internal combustion engine, for correct operation and proper functionality.
British patent specification GB 1 540 216 discloses a method of checking piezo-electric actuators in which the deflection of a valve tappet is measured with a coil. The measured value is compared in comparators with a reference value, and correct operation of the actuator is inferred on this basis. If the correct operation cannot be confirmed, a warning signal is output.
Additional information in this context may be gleaned from the German published patent application DE 198 04 196 A1, which is not a prior art publication. There is described a method of checking piezo-electric actuators, particularly for fuel-injection valves, in which the voltage on the actuator is compared with threshold values, and this is used to diagnose correct operation of the actuator.
German patent DE 196 52 809 discloses an assembly for driving a piezo-electric actuator for a fuel-injection valve in an internal combustion engine. Such an actuator can be regarded as a capacitive energy store.
In order to open a fuel-injection valve in an internal combustion engine, it is necessary to apply an electric charge to the actuator. When the charge is removed from the actuator, the injection valve is closed. The injected fuel quantity is primarily dependent, for a constant fuel pressure, for example in a common rail fuel-injection system, on the injection duration.
When any capacitive actuator is charged, a charging current flows into the actuator; the latter is charged when the charging current becomes zero. During the charging process, the actuator voltage across the actuator rises to a particular value. In the charged state, no current flows, and the actuator voltage remains approximately constant. During discharge, a discharge current flows out of the actuator. The latter is discharged when the discharge current returns to zero. During the discharging process, the actuator voltage drop across the actuator falls to zero volts.
These processes can be disrupted by internal or external disruptive influences such that the charge applied to the actuator remains on the actuator longer than stipulated by the control signals output by an engine control system, and the fuel-injection valve remains open for an undefined duration, which results in too much fuel being injected.
It is accordingly an object of the invention to provide a method of checking a capacitive actuator for proper operation, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for a method of driving the capacitive actuator and allows a simple way of monitoring actuator actuation independently of the drive signal.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of checking a capacitive actuator, in particular a fuel injection valve in an internal combustion engine, for correct operation. The method comprises the following steps:
operating a capacitive actuator with a control signal having a given duration defined by a start and an end; defining a duration of actuator actuation to extend from an instant at which a charging current produced by the start of the control signal is below a predetermined, upper threshold value, up to an instant, at which a discharge current produced by the end of the control signal is above a predetermined, lower threshold value;
measuring the duration of actuator actuation and comparing the duration with the given duration of the control signal; and
determining that the actuator is operating correctly if the measured duration deviates from the duration of the control signal only within an acceptable range.
In accordance with an alternative embodiment of the inventive concept there is provided method of checking a capacitive actuator for correct operation, which comprises:
operating a capacitive actuator with a control signal having a given duration from a start to an end;
defining a duration of actuator actuation to extend from an instant at which an actuator voltage, rising after the start of the control signal, lies above a predetermined threshold value, up to an instant at which the actuator voltage, falling after the end of the control signal, lies below the threshold value;
measuring the duration of actuator actuation and comparing the measured duration with the given duration of the control signal; and
determining that the actuator is operating correctly if the measured duration deviates from the duration of the control signal only within an acceptable range.
In other words, the current xc2x1Ip supplied to the actuator and dissipated by it or the voltage Up applied to the actuator is measured and, by comparison with threshold values, the actuation duration of the actuator (which is identical to the fuel-injection duration in the internal combustion engine context) is determined. This actuation duration ti is then compared with the duration tst of the control signal st output by an engine control unit. If the two signals ti and tst are the same within predetermined limits, fuel injection is assumed to be operating correctly. Faults can be recognized quickly.
In a preferred implementation, the acceptable range is defined as [tstxe2x88x92x] less than ti less than [tst+x], where tst is the duration of the control signal, ti is the measured duration of actuator actuation, and x is a predefined time period (e.g., 3% of tst).
In accordance with an additional feature of the invention, an optical error display and/or an entry in an error memory of a diagnostic circuit for the actuator is produced if ti less than [tstxe2x88x92x] or if ti greater than [tst+x].
In accordance with a concomitant feature of the invention, the actuator is turned off if ti less than [tstxe2x88x92x] or if ti greater than [tst+x].
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of checking a capacitive actuator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.